1. DNA polymerase molecules copying a template in vitro can produce nonrandom bursts of mutations, suggesting that a substantial minority of the polymerase molecules have a mutator phenotype. However, a polymerase must be processive to be able to produce such clusters. We therefore closely examined the ability of the mammalian DNA repair polymerase Pol beta to make clusters, as had been suggested in the literature. The putative clusters appear to have been artifacts of the measurement systems, confirming that processivity is required for cluster formation. This work has now been published. 2. We are now tempting to determine whether multiple mutations can arise simultaneously in Escherichia coli at sites separated by hundreds of kilobases, a property that would be required for clusters to promote carcinogenesis in mammalian cells. Although the measurement is conceptually simple, the tests have proven extraordinarily difficult to date. 3. Mutation rates are difficult to measure in riboviruses, the largest class of human pathogens. In particular, the question of whether riboviruses share a common genomic mutation rate has resisted resolution. After many efforts, we have recently established such a system using the phage Q-beta growing on Escherichia coli. Mutation rates are high as expected from a ribovirus and we have preliminary evidence for a basal mutation rate that is far more variable than in a bacterium. 4. Because we often use rI mutations in bacteriophage T4 as mutation reporters, we sequenced a set of rI-like mutations that did not map at the classical rI locus in order to determine where they did map. They occupied the gene upstream from rI, or were in the rIII, rIV or rV genes, but not elsewhere. Because the r genes are involved in lysis inhibition, a classical system of interest to students of evolution, this result sets resolves concerns about how many genes are involved. This work is now being prepared for publication.